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          Java多线程JUC原子类之AtomicReference
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<p>转载自<a target="_blank" rel="noopener" href="https://www.cnblogs.com/skywang12345/p/3514623.html">https://www.cnblogs.com/skywang12345/p/3514623.html</a></p>
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<h1 id="AtomicReference介绍和函数列表"><a href="#AtomicReference介绍和函数列表" class="headerlink" title="AtomicReference介绍和函数列表"></a>AtomicReference介绍和函数列表</h1><p>AtomicReference是作用是对”对象”进行原子操作。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 使用 null 初始值创建新的 AtomicReference。</span></span><br><span class="line">AtomicReference()</span><br><span class="line"><span class="comment">// 使用给定的初始值创建新的 AtomicReference。</span></span><br><span class="line">AtomicReference(V initialValue)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。</span></span><br><span class="line"><span class="function"><span class="keyword">boolean</span> <span class="title">compareAndSet</span><span class="params">(V expect, V update)</span></span></span><br><span class="line"><span class="function"><span class="comment">// 获取当前值。</span></span></span><br><span class="line"><span class="function">V <span class="title">get</span><span class="params">()</span></span></span><br><span class="line"><span class="function"><span class="comment">// 以原子方式设置为给定值，并返回旧值。</span></span></span><br><span class="line"><span class="function">V <span class="title">getAndSet</span><span class="params">(V newValue)</span></span></span><br><span class="line"><span class="function"><span class="comment">// 最终设置为给定值。</span></span></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">lazySet</span><span class="params">(V newValue)</span></span></span><br><span class="line"><span class="function"><span class="comment">// 设置为给定值。</span></span></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">set</span><span class="params">(V newValue)</span></span></span><br><span class="line"><span class="function"><span class="comment">// 返回当前值的字符串表示形式。</span></span></span><br><span class="line"><span class="function">String <span class="title">toString</span><span class="params">()</span></span></span><br><span class="line"><span class="function"><span class="comment">// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。</span></span></span><br><span class="line"><span class="function"><span class="keyword">boolean</span> <span class="title">weakCompareAndSet</span><span class="params">(V expect, V update)</span></span></span><br></pre></td></tr></table></figure>

<h1 id="源码（JDK1-8-0-202）"><a href="#源码（JDK1-8-0-202）" class="headerlink" title="源码（JDK1.8.0_202）"></a>源码（JDK1.8.0_202）</h1><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span 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class="line">148</span><br><span class="line">149</span><br><span class="line">150</span><br><span class="line">151</span><br><span class="line">152</span><br><span class="line">153</span><br><span class="line">154</span><br><span class="line">155</span><br><span class="line">156</span><br><span class="line">157</span><br><span class="line">158</span><br><span class="line">159</span><br><span class="line">160</span><br><span class="line">161</span><br><span class="line">162</span><br><span class="line">163</span><br><span class="line">164</span><br><span class="line">165</span><br><span class="line">166</span><br><span class="line">167</span><br><span class="line">168</span><br><span class="line">169</span><br><span class="line">170</span><br><span class="line">171</span><br><span class="line">172</span><br><span class="line">173</span><br><span class="line">174</span><br><span class="line">175</span><br><span class="line">176</span><br><span class="line">177</span><br><span class="line">178</span><br><span class="line">179</span><br><span class="line">180</span><br><span class="line">181</span><br><span class="line">182</span><br><span class="line">183</span><br><span class="line">184</span><br><span class="line">185</span><br><span class="line">186</span><br><span class="line">187</span><br><span class="line">188</span><br><span class="line">189</span><br><span class="line">190</span><br><span class="line">191</span><br><span class="line">192</span><br><span class="line">193</span><br><span class="line">194</span><br><span class="line">195</span><br><span class="line">196</span><br><span class="line">197</span><br><span class="line">198</span><br><span class="line">199</span><br><span class="line">200</span><br><span class="line">201</span><br><span class="line">202</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * An object reference that may be updated atomically. See the &#123;<span class="doctag">@link</span></span></span><br><span class="line"><span class="comment"> * java.util.concurrent.atomic&#125; package specification for description</span></span><br><span class="line"><span class="comment"> * of the properties of atomic variables.</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@since</span> 1.5</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> Doug Lea</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@param</span> &lt;V&gt; The type of object referred to by this reference</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">AtomicReference</span>&lt;<span class="title">V</span>&gt; <span class="keyword">implements</span> <span class="title">java</span>.<span class="title">io</span>.<span class="title">Serializable</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">long</span> serialVersionUID = -<span class="number">1848883965231344442L</span>;</span><br><span class="line"></span><br><span class="line">    <span class="comment">// 获取Unsafe对象，Unsafe的作用是提供CAS操作</span></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> Unsafe unsafe = Unsafe.getUnsafe();</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">long</span> valueOffset;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">static</span> &#123;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            valueOffset = unsafe.objectFieldOffset</span><br><span class="line">                (AtomicReference.class.getDeclaredField(<span class="string">&quot;value&quot;</span>));</span><br><span class="line">        &#125; <span class="keyword">catch</span> (Exception ex) &#123; <span class="keyword">throw</span> <span class="keyword">new</span> Error(ex); &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">// volatile类型</span></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">volatile</span> V value;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Creates a new AtomicReference with the given initial value.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> initialValue the initial value</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">AtomicReference</span><span class="params">(V initialValue)</span> </span>&#123;</span><br><span class="line">        value = initialValue;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Creates a new AtomicReference with null initial value.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">AtomicReference</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Gets the current value.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the current value</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> V <span class="title">get</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> value;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Sets to the given value.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> newValue the new value</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">set</span><span class="params">(V newValue)</span> </span>&#123;</span><br><span class="line">        value = newValue;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Eventually sets to the given value.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> newValue the new value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@since</span> 1.6</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">lazySet</span><span class="params">(V newValue)</span> </span>&#123;</span><br><span class="line">        unsafe.putOrderedObject(<span class="keyword">this</span>, valueOffset, newValue);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically sets the value to the given updated value</span></span><br><span class="line"><span class="comment">     * if the current value &#123;<span class="doctag">@code</span> ==&#125; the expected value.</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> expect the expected value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> update the new value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> &#123;<span class="doctag">@code</span> true&#125; if successful. False return indicates that</span></span><br><span class="line"><span class="comment">     * the actual value was not equal to the expected value.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">boolean</span> <span class="title">compareAndSet</span><span class="params">(V expect, V update)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> unsafe.compareAndSwapObject(<span class="keyword">this</span>, valueOffset, expect, update);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically sets the value to the given updated value</span></span><br><span class="line"><span class="comment">     * if the current value &#123;<span class="doctag">@code</span> ==&#125; the expected value.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * &lt;p&gt;&lt;a href=&quot;package-summary.html#weakCompareAndSet&quot;&gt;May fail</span></span><br><span class="line"><span class="comment">     * spuriously and does not provide ordering guarantees&lt;/a&gt;, so is</span></span><br><span class="line"><span class="comment">     * only rarely an appropriate alternative to &#123;<span class="doctag">@code</span> compareAndSet&#125;.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> expect the expected value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> update the new value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> &#123;<span class="doctag">@code</span> true&#125; if successful</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">boolean</span> <span class="title">weakCompareAndSet</span><span class="params">(V expect, V update)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> unsafe.compareAndSwapObject(<span class="keyword">this</span>, valueOffset, expect, update);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically sets to the given value and returns the old value.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> newValue the new value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the previous value</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="meta">@SuppressWarnings(&quot;unchecked&quot;)</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> V <span class="title">getAndSet</span><span class="params">(V newValue)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> (V)unsafe.getAndSetObject(<span class="keyword">this</span>, valueOffset, newValue);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically updates the current value with the results of</span></span><br><span class="line"><span class="comment">     * applying the given function, returning the previous value. The</span></span><br><span class="line"><span class="comment">     * function should be side-effect-free, since it may be re-applied</span></span><br><span class="line"><span class="comment">     * when attempted updates fail due to contention among threads.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> updateFunction a side-effect-free function</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the previous value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@since</span> 1.8</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> V <span class="title">getAndUpdate</span><span class="params">(UnaryOperator&lt;V&gt; updateFunction)</span> </span>&#123;</span><br><span class="line">        V prev, next;</span><br><span class="line">        <span class="keyword">do</span> &#123;</span><br><span class="line">            prev = get();</span><br><span class="line">            next = updateFunction.apply(prev);</span><br><span class="line">        &#125; <span class="keyword">while</span> (!compareAndSet(prev, next));</span><br><span class="line">        <span class="keyword">return</span> prev;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically updates the current value with the results of</span></span><br><span class="line"><span class="comment">     * applying the given function, returning the updated value. The</span></span><br><span class="line"><span class="comment">     * function should be side-effect-free, since it may be re-applied</span></span><br><span class="line"><span class="comment">     * when attempted updates fail due to contention among threads.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> updateFunction a side-effect-free function</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the updated value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@since</span> 1.8</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> V <span class="title">updateAndGet</span><span class="params">(UnaryOperator&lt;V&gt; updateFunction)</span> </span>&#123;</span><br><span class="line">        V prev, next;</span><br><span class="line">        <span class="keyword">do</span> &#123;</span><br><span class="line">            prev = get();</span><br><span class="line">            next = updateFunction.apply(prev);</span><br><span class="line">        &#125; <span class="keyword">while</span> (!compareAndSet(prev, next));</span><br><span class="line">        <span class="keyword">return</span> next;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically updates the current value with the results of</span></span><br><span class="line"><span class="comment">     * applying the given function to the current and given values,</span></span><br><span class="line"><span class="comment">     * returning the previous value. The function should be</span></span><br><span class="line"><span class="comment">     * side-effect-free, since it may be re-applied when attempted</span></span><br><span class="line"><span class="comment">     * updates fail due to contention among threads.  The function</span></span><br><span class="line"><span class="comment">     * is applied with the current value as its first argument,</span></span><br><span class="line"><span class="comment">     * and the given update as the second argument.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> x the update value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> accumulatorFunction a side-effect-free function of two arguments</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the previous value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@since</span> 1.8</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> V <span class="title">getAndAccumulate</span><span class="params">(V x,</span></span></span><br><span class="line"><span class="function"><span class="params">                                    BinaryOperator&lt;V&gt; accumulatorFunction)</span> </span>&#123;</span><br><span class="line">        V prev, next;</span><br><span class="line">        <span class="keyword">do</span> &#123;</span><br><span class="line">            prev = get();</span><br><span class="line">            next = accumulatorFunction.apply(prev, x);</span><br><span class="line">        &#125; <span class="keyword">while</span> (!compareAndSet(prev, next));</span><br><span class="line">        <span class="keyword">return</span> prev;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Atomically updates the current value with the results of</span></span><br><span class="line"><span class="comment">     * applying the given function to the current and given values,</span></span><br><span class="line"><span class="comment">     * returning the updated value. The function should be</span></span><br><span class="line"><span class="comment">     * side-effect-free, since it may be re-applied when attempted</span></span><br><span class="line"><span class="comment">     * updates fail due to contention among threads.  The function</span></span><br><span class="line"><span class="comment">     * is applied with the current value as its first argument,</span></span><br><span class="line"><span class="comment">     * and the given update as the second argument.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> x the update value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> accumulatorFunction a side-effect-free function of two arguments</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the updated value</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@since</span> 1.8</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">final</span> V <span class="title">accumulateAndGet</span><span class="params">(V x,</span></span></span><br><span class="line"><span class="function"><span class="params">                                    BinaryOperator&lt;V&gt; accumulatorFunction)</span> </span>&#123;</span><br><span class="line">        V prev, next;</span><br><span class="line">        <span class="keyword">do</span> &#123;</span><br><span class="line">            prev = get();</span><br><span class="line">            next = accumulatorFunction.apply(prev, x);</span><br><span class="line">        &#125; <span class="keyword">while</span> (!compareAndSet(prev, next));</span><br><span class="line">        <span class="keyword">return</span> next;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Returns the String representation of the current value.</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span> the String representation of the current value</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> String <span class="title">toString</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> String.valueOf(get());</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><strong>说明</strong>：AtomicReference的源码比较简单。它是通过”volatile”和”Unsafe提供的CAS函数实现”原子操作。(01) value是volatile类型。这保证了：当某线程修改value的值时，其他线程看到的value值都是最新的value值，即修改之后的volatile的值。(02) 通过CAS设置value。这保证了：当某线程池通过CAS函数(如compareAndSet函数)设置value时，它的操作是原子的，即线程在操作value时不会被中断。</p>
<h1 id="AtomicReference示例"><a href="#AtomicReference示例" class="headerlink" title="AtomicReference示例"></a>AtomicReference示例</h1><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// AtomicReferenceTest.java的源码</span></span><br><span class="line"><span class="keyword">import</span> java.util.concurrent.atomic.AtomicReference;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">AtomicReferenceTest</span> </span>&#123;</span><br><span class="line">    </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span>&#123;</span><br><span class="line"></span><br><span class="line">        <span class="comment">// 创建两个Person对象，它们的id分别是101和102。</span></span><br><span class="line">        Person p1 = <span class="keyword">new</span> Person(<span class="number">101</span>);</span><br><span class="line">        Person p2 = <span class="keyword">new</span> Person(<span class="number">102</span>);</span><br><span class="line">        <span class="comment">// 新建AtomicReference对象，初始化它的值为p1对象</span></span><br><span class="line">        AtomicReference ar = <span class="keyword">new</span> AtomicReference(p1);</span><br><span class="line">        <span class="comment">// 通过CAS设置ar。如果ar的值为p1的话，则将其设置为p2。</span></span><br><span class="line">        ar.compareAndSet(p1, p2);</span><br><span class="line"></span><br><span class="line">        Person p3 = (Person)ar.get();</span><br><span class="line">        System.out.println(<span class="string">&quot;p3 is &quot;</span>+p3);</span><br><span class="line">        System.out.println(<span class="string">&quot;p3.equals(p1)=&quot;</span>+p3.equals(p1));</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Person</span> </span>&#123;</span><br><span class="line">    <span class="keyword">volatile</span> <span class="keyword">long</span> id;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Person</span><span class="params">(<span class="keyword">long</span> id)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.id = id;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> String <span class="title">toString</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="string">&quot;id:&quot;</span>+id;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><strong>运行结果</strong>：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">p3 is id:<span class="number">102</span></span><br><span class="line">p3.equals(p1)=<span class="keyword">false</span></span><br></pre></td></tr></table></figure>

<p><strong>结果说明</strong>：新建AtomicReference对象ar时，将它初始化为p1。紧接着，通过CAS函数对它进行设置。如果ar的值为p1的话，则将其设置为p2。最后，获取ar对应的对象，并打印结果。p3.equals(p1)的结果为false，这是因为Person并没有覆盖equals()方法，而是采用继承自Object.java的equals()方法；而Object.java中的equals()实际上是调用”==”去比较两个对象，即比较两个对象的地址是否相等。</p>

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